1. Field of the Invention
The present invention relates to a video system for mixing a video signal, and more particularly, to a video signal mixing apparatus and method for mixing two video signals while minimizing jitter.
2. Description of the Related Art
FIG. 1 shows a displayed image mixed by a conventional video signal mixing method. Here, the X-axis represents pixels in the horizontal direction, and the Y-axis represents horizontal lines in the vertical directions. FIG. 1 shows a partial screen having 50 pixels.times.100 horizontal lines.
A mixed image displayed in a video system includes a main screen 12 and a sub-screen 14. It is assumed that a video signal (or an internal video signal) on the main screen 12 is generated by a system A and a video signal (or external video signal) on the sub-screen 14 is generated by a system B.
Here, the internal video signal and external video signal are synchronized with each other based on a video synchronous signal. The video synchronous signal means a horizontal synchronous signal and a vertical synchronous signal. However, the internal and external video signals are not processed using the same system clock signal. That is, the internal video signal is synchronized with a system clock signal (or internal clock signal) of the system A while the external video signal is synchronized with a system clock signal (or external clock signal) of the system B.
When the system A mixes the internal and external video signals in response to a digital selection signal synchronized with the external clock signal, timing jitter occurs in the mixed video signal. This is because a digital selection signal is input to the system A after being delayed by 1 period of the internal clock signal or advanced by 1 period of the internal clock signal, so the point in time that the system A recognizes the digital selection signal has an error of about 1 period of the internal clock signal.
For example, assuming that one period of the internal clock signal is 100 ns, the timing jitter of about 100 ns occurs. That is, whenever the digital selection signal is input to the system A within 1 period of the internal clock signal, timing jitter of 100 ns occurs in the resultant mixed video image.
In a video system having a picture in picture (PIP) function, the above-described timing jitter appears as dot crawl at a vertical boundary 10 by 1 period of the internal clock signal, shown in FIG. 1.
Therefore, a need exists to provide a video signal mixing apparatus capable of minimizing timing jitter caused when digitally mixing video signals which have not been synchronized with each other.